Negative electrode for non-aqueous electrolyte secondary battery and non-aqueous electrolyte secondary battery comprising the same

ABSTRACT

A negative electrode containing an alloying material and a graphite material for providing a non-aqueous electrolyte secondary battery with a high capacity and excellent cycle characteristics. The negative electrode includes graphite and at least one alloying material capable of electrochemically absorbing and desorbing Li. The alloying material includes an A phase composed mainly of Si and a B phase including an intermetallic compound of at least one transition metal element and Si. At least one of the A phase and the B phase includes a microcrystalline or amorphous region. The weight percentage of the A phase relative to the total weight of the A phase and the B phase is greater than 40% and not greater than 95%. The weight percentage of the graphite relative to the total weight of the alloying material and the graphite is not less than 50% and not greater than 95%.

FIELD OF THE INVENTION

The present invention relates to a non-aqueous electrolyte secondarybattery with a high capacity and a long life. More specifically, thepresent invention relates to an improvement in the negative electrodefor a non-aqueous electrolyte secondary battery.

BACKGROUND OF THE INVENTION

An extensive research and development has been conducted on the use oflithium metal, which is capable of realizing high voltage and highenergy density, as the negative electrode of non-aqueous electrolytesecondary batteries. This has led to the current commercialization oflithium ion batteries that use a graphite material in the negativeelectrode, which material reversibly absorbs and desorbs lithium andprovides good cycle life and safety.

However, the practical (i.e., useful) capacity of batteries using agraphite material-based negative electrode is approximately 350 mAh/g,which is very close to the 372 mAh/g theoretical capacity of thegraphite material. Therefore, as long as a graphite material is used inthe negative electrode, it is not feasible to achieve a dramaticincrease in capacity. Meanwhile, as more and more portable appliancesbecome available, non-aqueous electrolyte secondary batteries used asthe energy source of such appliances are required to have highercapacities. Accordingly, in order to achieve higher capacities, negativeelectrode materials having a higher capacity than graphite becomenecessary.

Alloy forming materials (hereinafter referred to as “alloyingmaterials”) containing silicon or tin are currently receiving attentionas the materials that offer a higher capacity. Metal elements, such assilicon and tin, are capable of electrochemically absorbing anddesorbing lithium ions, thereby enabling a very large capacity chargeand discharge in comparison with graphite materials. For example, it isknown that silicon has a theoretical discharge capacity of 4199 mAh/g,which is 11 times higher than that of graphite.

When an alloying material absorbs lithium, it forms a lithium alloy,such as a lithium-silicon alloy or a lithium-tin alloy. The formation ofa lithium alloy involves a very large expansion caused by the change inits crystal structure. For example, the volume of silicon theoreticallyexpands 4.1-fold when it absorbs lithium to its maximum. As a result,the active material, i.e., the alloyed material, separates and falls offthe current collector of the negative electrode, thereby resulting inloss of electrical conduction and a degradation in batterycharacteristics, particularly high-rate discharge characteristics andcharge and discharge (hereinafter referred to as “charge/discharge”)cycle characteristics. In the case of graphite, its volume expands only1.1-fold, because lithium is intercalated between the layers of graphite(intercalation reaction).

In order to lessen such expansion and obtain higher capacities, the useof a combination of graphite and an alloying material has extensivelybeen attempted. However, when graphite and an alloying material aresimply mixed, the alloying material expands in uneven directions in theelectrode plate, so that the graphite particles around the alloyingmaterial are moved by the stress exerted by the expansion of thealloying material, thereby resulting in separation. Consequently, theelectronic conductivity lowers and the high-rate dischargecharacteristics and charge/discharge cycle characteristics of theresultant battery deteriorate, in the same manner as the negativeelectrode including an alloy material alone.

Japanese Laid-Open Patent Publication No. 2000-357515 proposescontrolling the ratio of the particle size RSi of a silicide to theparticle size Rc of a carbon material, i.e., the RSi/Rc ratio, to 1 orless, in order to lessen the impact of large expansion of the alloyingmaterial and improve battery characteristics. However, even if suchparticle size control can lessen the impact of alloy expansion, itcannot suppress the degradation of current collecting property caused bycracking of particles of alloying material and the like. Also,charge/discharge cycles cause particles of alloying material to becomecracked, thereby increasing the surface area of the alloy material.Thus, there is also a problem of side reaction, i.e., formation of acoating film on the surface of the alloy. Accordingly, this proposal isnot practical.

Japanese Laid-Open Patent Publication No. 2000-243396 proposesembedding, in a carbon particle, a metal particle or a metal oxideparticle that is capable of electrochemically reacting with Li.According to this proposal, by fixing the metal particle or metal oxideparticle to the surface of the carbon particle, the separation of themetal or metal oxide particle due to its expansion is suppressed. Inthis case, this proposal is highly effective in the initial stage ofcharge and discharge cycles, but repetitive expansion and contractioncauses the metal particle or metal oxide particle to separate from thecarbon particle. As a result, the expansion rate of the negativeelectrode increases, and separation occurs throughout the electrodeplate.

As described above, in order to make full use of a high capacityalloying material as a negative electrode material, the use of acombination of an alloying material and a graphite material has beenextensively examined, but no proposal has succeeded in sufficientlyreducing the impact of uneven expansion of the alloy material.Specifically, according to conventional proposals, the electricalconduction between particles in a negative electrode is broken, and analloying material and a graphite material separate from a currentcollector. Consequently, the electronic conductivity of the negativeelectrode lowers, leading to a degradation in battery characteristics.

BRIEF SUMMARY OF THE INVENTION

In view of the above problems which occur when a graphite material andan alloying material that contains Si and is capable ofelectrochemically absorbing and desorbing Li are used as activematerials, the present invention has been made with the aim ofsuppressing the deterioration of battery characteristics caused by theabove-described expansion of the alloying material.

The present invention relates to a negative electrode for a non-aqueouselectrolyte secondary battery, comprising graphite and at least onealloying material capable of electrochemically absorbing and desorbingLi. The alloying material includes an A phase composed mainly of Si anda B phase including an intermetallic compound of at least one transitionmetal element and Si. At least one of the A phase and the B phaseincludes a microcrystalline or amorphous region. The weight percentageof the A phase relative to the total weight of the A phase and the Bphase is greater than 40% and not greater than 95%. The weightpercentage of the graphite relative to the total weight of the alloyingmaterial and the graphite is not less than 50% and not greater than 95%.

The alloying material desirably exists in gaps between particles of thegraphite.

The alloying material desirably has a maximum particle size of 10 μm orless.

At least a part of the alloying material is desirably adhered to thesurface of the graphite via a binder.

The ratio of the mean particle size of the alloying material to the meanparticle size of the graphite is desirably in the range of 0.15 to 0.90.

The negative electrode according to the present invention can furtherinclude an auxiliary conductive agent. The auxiliary conductive agentdesirably has a specific surface area of 10 m²/g or more.

The auxiliary conductive agent desirably comprises carbon fibers havingan aspect ratio of 10 or more. Desirably, at least one end of the carbonfibers is adhered or bonded to the alloying material or is adhered orbonded to the graphite.

It is particularly preferable that one end of at least a part of thecarbon fibers be adhered or bonded to the alloying material while theother end be adhered or bonded to the graphite.

The carbon fibers are obtained by heating at least one of the alloyingmaterial and the graphite in the flow of hydrocarbon gas.

The weight percentage of the auxiliary conductive agent relative to thetotal weight of the alloy material, the graphite and the auxiliaryconductive agent is desirably 10% or less.

The present invention also pertains to a non-aqueous electrolytesecondary battery comprising a positive electrode capable ofelectrochemically absorbing and desorbing Li, a negative electrode and anon-aqueous electrolyte, wherein the negative electrode includesgraphite and at least one alloying material capable of electrochemicallyabsorbing and desorbing Li, the alloying material includes an A phasecomposed mainly of Si and a B phase including an intermetallic compoundof at least one transition metal element and Si, at least one of the Aphase and the B phase includes a microcrystalline or amorphous region,the weight percentage of the A phase relative to the total weight of theA phase and the B phase is greater than 40% and not greater than 95%,and the weight percentage of the graphite relative to the total weightof the alloying material and the graphite is not less than 50% and notgreater than 95%.

According to the present invention, in a negative electrode employing acombination of an alloying material and a graphite material, thedeterioration of battery characteristics due to the expansion of thealloy material can be suppressed. Therefore, it is possible to realize anon-aqueous electrolyte secondary battery with a high capacity andexcellent cycle characteristics.

While the novel features of the invention are set forth particularly inthe appended claims, the invention, both as to organization and content,will be better understood and appreciated, along with other objects andfeatures thereof, from the following detailed description taken inconjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a photograph of a section of one exemplary negative electrodein accordance with the present invention (at 1000 magnification);

FIG. 2 is an XRD profile of a Ti—Si alloying material in accordance withthe present invention;

FIG. 3 is the XRD profile of FIG. 2 from which the background isremoved; and

FIG. 4 is a longitudinal sectional view of a cylindrical batteryprepared in examples of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

An alloying material capable of electrochemically absorbing anddesorbing Li according to the present invention has differentcharacteristics from those of conventional alloying materials. Thealloying material of the present invention includes an A phase composedmainly of Si and a B phase comprising an intermetallic compound of atransition metal element and Si. This alloying material lessens theexpansion and suppresses the degradation of the electronic conductivityof the resultant negative electrode due to expansion and contraction.Therefore, a negative electrode for a non-aqueous electrolyte secondarybattery according to the present invention including this alloyingmaterial and graphite gives a battery having a high capacity andexcellent cycle characteristics.

The A phase is a phase that absorbs and desorbs Li, being capable ofelectrochemically reacting with Li. The A phase may be composed mainlyof Si, and is preferably composed simply of Si. When the A phase iscomposed only of Si, the alloying material can absorb and desorb anextremely large amount of Li per unit weight or unit volume. However,since Si itself is a semiconductor, it has poor electronic conductivity.It is therefore effective to make the A phase include a small amount ofan element, such as phosphorus (P) or hydrogen (H), or a transitionmetal element, in an amount up to about 5% by weight.

The B phase comprises an intermetallic compound of a transition metalelement and Si. Since an intermetallic compound containing Si has a highaffinity for the A phase, cracking is unlikely to occur at the interfacebetween the A phase and the B phase. Also, the B phase has a higherelectronic conductivity and a higher hardness than the phase composedmainly of Si. Accordingly, the B phase compensates for the poorelectronic conductivity of the A phase and resists expansion stress soas to maintain the shape of the particles of alloying material. Two ormore kinds of B phases may be present. That is, two or more kinds ofintermetallic compounds each having a different composition may bepresent as the B phases. For example, MSi₂ and MSi (M is a transitionmetal) may be present in the particles of alloying material. Also,intermetallic compounds each having a different transition metalelement, such as M¹Si₂ and M²Si₂ (M¹ and M² are different transitionmetals), may be present in the particles of alloying material.

The transition metal element in the B phase is preferably at least oneselected from the group consisting of Ti, Zr, Ni, Co, Mn, Fe, and Cu,and more preferably at least one selected from the group consisting ofTi and Zr. Silicides of these elements have a higher electronicconductivity and a higher hardness than silicides of other elements.

The A phase and/or the B phase comprise a microcrystalline or amorphousregion. If a crystalline alloying material is used, particles thereoftend to become cracked upon Li absorption, which causes a rapiddeterioration in the current collecting property of the negativeelectrode, thereby resulting in a degradation in batterycharacteristics. In contrast, if a microcrystalline or amorphousalloying material is used, particles thereof are unlikely to becomecracked upon expansion due to Li absorption.

According to the present invention, when the diameter of crystal grains(crystallites) of an alloying material is 50 nm or less, the alloyingmaterial is defined as microcrystalline. When an alloying material has amicrocrystalline region, the X-ray diffraction spectrum of the particlesof alloying material shows one or more peaks that are, although notsharp, relatively clear such that the half width is recognizable. Thediameter of crystal grains (crystallites) of an alloying material iscalculated from the half width of the most intensive peak in the X raydiffraction spectrum of the particles of the alloying material, and theScherrer equation.

On the other hand, if an alloying material comprises an amorphousregion, the X-ray diffraction spectrum of particles of such an alloyingmaterial shows a broad halo pattern in the range of 2θ=15 to 40° suchthat the half width is not recognizable.

The weight percent of the A phase relative to the total weight of the Aphase and the B phase is greater than 40% and not greater than 95%. Whenthe weight percent of the A phase is greater than 40%, a high capacitycan be achieved effectively. Also, when the weight percent of the Aphase is 95% or less, the low electronic conductivity of the A phase canbe compensated for by the B phase, the shape of particles of alloyingmaterial can be maintained effectively, and the particles of alloyingmaterial can be easily made microcrystalline or amorphous. From theviewpoint of making these effects more remarkable, the weight percent ofthe A phase relative to the total weight of the A phase and the B phaseis desirably not less than 65% and not more than 85%, and morepreferably not less than 70% and not more than 80%.

Also, if the weight percent of the A phase exceeds 95%, it becomesdifficult to make the particles of alloying material microcrystalline oramorphous, which is not suitable for the present invention. Conversely,if the weight percent of the A phase is less than 40%, the resultantbattery has a lower capacity than conventional batteries using graphitein the negative electrode, which is not suitable for the presentinvention.

The Si content in the alloying material according to the presentinvention is preferably 60% by weight or more. When the weight percentof Si relative to the whole alloy is 60% or more, Si and the transitionmetal form an intermetallic compound (silicide) such that the weightpercent of the A phase exceeds 40%. Therefore, it becomes possible toeffectively realize a high capacity.

The negative electrode of the present invention contains theabove-described alloying material and graphite. From the viewpoint oftaking advantage of the abilities of both the alloying material and thegraphite in good balance, the weight percent of the graphite relative tothe total weight of the alloying material and the graphite is not lessthan 50% and not greater than 95%, and preferably not less than 65% andnot greater than 85%. If the graphite is less than 50% by weight, theamount of the graphite contacting the alloying material decreases, andthe contact between the particles of alloying material increases. As aresult, the expansion of the alloying material is apt to create gapsinside the negative electrode, thereby increasing the expansion of thewhole negative electrode. On the other hand, if the graphite is morethan 95% by weight, the contribution of the alloying material to thecapacity becomes extremely small, so that the capacity of the resultantelectrode is almost equivalent to that of a negative electrode usinggraphite alone.

The graphite used in the present invention may be any graphite materialthat can be generally used in non-aqueous electrolyte secondarybatteries. For example, natural graphites, such as flake graphite, andartificial graphites produced by various methods may be used.

The mean particle size of the graphite is preferably not less than 5 μmand not more than 50 μm, and more preferably not less than 7 μm and notmore than 25 μm. If the mean particle size of the graphite is too small,the ability to accommodate deformation increases, thereby lessening theadverse effect of the alloying material expansion on the electrodeplate, but the specific surface area of the graphite itself increases.In order to suppress the side reaction between the graphite and anelectrolyte or the like, reduce the formation of a coating film on thegraphite surface, and minimize the irreversible capacity of the negativeelectrode, it is desirable that the mean particle size of the graphitebe 5 μm or more so as not to increase the specific surface area of thegraphite itself. Also, if the mean particle size of the graphite islarger than 50 μm, the surface of the resultant negative electrode tendsto become roughened, and the gaps inside the negative electrode becomelarger. Thus, it becomes difficult to collect current from the alloyingmaterial inside the negative electrode. From the viewpoint of obtaininga negative electrode with excellent current collecting property, it isdesirable that the mean particle size of the graphite be 50 μm or less.

Generally, the negative electrode for non-aqueous electrolyte secondarybatteries comprises a metal foil current collector and a negativeelectrode material mixture layer formed on each side of the currentcollector. Thus, the particle size of the graphite and the alloyingmaterial may be set such that it is smaller than the thickness of thenegative electrode material mixture layer on one side.

It is desirable that the alloying material exist in gaps between theparticles of the graphite. FIG. 1 is a photograph of a section of anexemplary negative electrode of the present invention. The negativeelectrode of FIG. 1 comprises a current collector 1 and a mixed materiallayer carried on one side of the current collector 1. The mixed materiallayer includes graphite particles 3 with a large particle size andparticles of alloying material 2 that are positioned so as to fill thegaps between the graphite particles 3. Also, there are suitable gapsaround the particles of alloying material 2. Accordingly, when theparticles 2 expand, such a structure can lessen the expansion andfacilitate collection of current upon the expansion. Also, thisstructure makes it possible to make the alloying material fully contactan electrolyte, leading to an improvement in high-rate dischargecharacteristics and charge/discharge cycle characteristics.

In order to easily obtain the structure as shown in FIG. 1, the maximumparticle size of the alloying material is desirably 10 μm or less, andmore preferably 5 μm or less. If the maximum particle size of thealloying material is larger than 10 μm, the proportion of the particlesof alloying material entering between the graphite particles decreases,and the proportion of the particles of alloying material thatagglomerate increases. If the particles of the alloy materialagglomerate, they push against one another when they expand, which maycause the negative electrode to expand excessively.

Also, at least a part of the alloying material is desirably adhered tothe surface of the graphite via a binder. Such a structure is excellentin terms of easing excessive expansion and maintaining the currentcollecting property. With such a structure, even if the alloyingmaterial expands and contracts repeatedly, the alloying material can bestably present in the gaps between the graphite particles. As a result,excessive expansion of the negative electrode material mixture layer issuppressed. Further, since the alloying material is fixed to thesurfaces of the graphite particles, current collection can be constantlyensured.

In order to obtain such a structure, it is desirable to use a techniqueof mixing graphite and a binder and then adding the alloying materialthereto and further mixing them. Since graphite has almost no functionalgroup on the surface thereof, it has a low affinity for the binder. Itis therefore desirable to mix graphite and the binder while applying astrong stirring force or stress thereto in advance. On the other hand,since the surface of the alloying material is generally covered with anoxide or the like, it has a high affinity for the binder. Hence, theabove-mentioned structure can be obtained only by mixing the alloyingmaterial with a mixture of graphite and the binder.

In order to easily obtain the structure as illustrated in FIG. 1, theratio of the mean particle size of the alloying material (R alloy) andthe mean particle size of the graphite (R graphite), i.e., the R alloy/Rgraphite ratio, is desirably in the range of 0.15 to 0.90. For example,when the mean particle size of the graphite (R graphite) is 18 μm, thepreferable range of the mean particle size of the alloying material (Ralloy) is 2.7 μm to 16.2 μm. However, as mentioned above, it is moredesirable that the maximum particle size of the alloying material be 10μm or less. Accordingly, the optimum range of R alloy is not less than2.7 μm and not more than 10 μm.

If the R alloy/R graphite ratio is less than 0.15, a large number ofparticles of the alloying material tend to be squeezed into the gapsbetween the graphite particles. Thus, when the particles of the alloyingmaterial expand, they push against one another, which may cause thenegative electrode to expand relatively largely. On the other hand, ifthe R alloy/R graphite ratio is greater than 0.9, the size of thegraphite particles is almost equal to the size of the particles ofalloying material. Thus, the gaps inside the negative electrode, whichserve to lessen the expansion of the particles of alloying material,decrease. The most preferable range of the R alloy/R graphite ratio is0.2 to 0.4. In this range, the ability to lessen the expansion of thealloying material becomes highest.

The negative electrode can further include an auxiliary conductiveagent. The auxiliary conductive agent is mainly added to improve theefficiency of collecting current from the alloy material. It istherefore preferred that the auxiliary conductive agent exist mainlynear the particles of alloying material.

The specific surface area of the auxiliary conductive agent is desirably10 m²/g or more. Although an auxiliary conductive agent having aspecific surface area of less than 10 m²/g can improve thecurrent-collecting efficiency, it is desirable to use an auxiliaryconductive agent having a specific surface area of 10 m²/g or more, inorder to obtain the effect of improving the current-collectingefficiency by using a small amount thereof. Preferable examples of theauxiliary conductive agent include carbon blacks, and among them,acetylene black is preferred. Also, carbon fibers having an aspect ratioof 10 or more are also preferable as the auxiliary conductive agent.Particularly, the carbon fibers contribute to the maintenance of currentcollecting property between the alloying material particles or betweenthe alloying material and the graphite.

At least one end of the carbon fibers is desirably adhered to thesurface of the alloying material or graphite, and is particularlydesirably bonded (for example, chemically bonded) to the surface of thealloying material or graphite. In this case, even when the alloyingmaterial is expanding or contracting, electrons can be stably donatedand accepted via the carbon fibers. As a result, the current collectingproperty is improved. It is particularly desirable that one end of thecarbon fiber be adhered or bonded to the alloying material while theother end be adhered or bonded to the graphite present nearby. When thecarbon fiber is bonded to both the alloying material and the graphite,the alloying material can be fixed to the graphite surface more firmly.Hence, the effect of easing the negative electrode expansion increases.

The carbon fibers may be vapor-phase growth carbon fibers (VGCF) orcarbon nanotubes. For example, by mixing and kneading carbon fibers andat least one of the alloying material and the graphite with a binder,the carbon fibers can be included in the negative electrode as theauxiliary conductive agent. Also, the carbon fibers can be grown on thesurface of the alloying material and/or graphite by heating at least oneof the alloying material and the graphite in the flow of hydrocarbongas. The heating atmosphere is desirably a reducing atmosphere. As thehydrocarbon, for example, methane, ethane, ethylene, acetylene, etc.,can be used. The heating temperature is preferably 400 to 800° C.Instead of hydrocarbon, carbon monoxide may be used. If the heatingtemperature is lower than 400° C., a sufficient amount of carbon fibersmay not be produced, thereby making the conductivity of the negativeelectrode insufficient. On the other hand, if the heating temperature ishigher than 800° C., highly conductive carbon fibers are produced, butthe crystallization of the alloying material may proceed, resulting indegradation in electrode characteristics.

In order to suppress the decrease in discharge capacity and the increasein irreversible capacity due to side reaction, the ratio of theauxiliary conductive agent to the total weight of the alloying material,the graphite and the auxiliary conductive agent is desirably 10% byweight or less, and more desirably 5% by weight or less.

The negative electrode includes a binder which binds the graphitematerial and the alloying material together and fix a mixed materiallayer to a current collector. The binder is selected to be a materialthat is electrochemically inactive with respect to Li in the potentialrange of the negative electrode and that has as little effect aspossible on other substances. Suitable examples of the binder includestyrene-butadiene copolymer rubber, polyacrylic acid, polyethylene,polyurethane, polymethyl methacrylate, polyvinylidene fluoride,polytetrafluoroethylene, carboxymethyl cellulose, and methyl cellulose.They may be used singly or in combination of two or more of them. Withrespect to the amount of the binder to be added, more is preferable, interms of maintaining the structure of the mixed material mixture layer;however, in terms of enhancing battery capacity and dischargecharacteristics, less is preferable.

When the negative electrode of the present invention comprises a currentcollector made of a metal foil and a negative electrode mixed materiallayer carried on each side of the current collector, it is desirable touse copper foil or copper alloy foil as the current collector. In usingcopper alloy foil, the copper content is preferably 90% by weight ormore. In order to improve the strength or flexibility of the currentcollector, it is effective that the current collector contain an elementsuch as P, Ag, or Cr.

The thickness of the current collector is preferably not less than 6 μmand not greater than 40 μm. If the current collector has a thickness ofless than 6 μm, it is difficult to handle and lacks sufficient strength,so that it may become broken or wrinkled when the material mixture layerexpands and contracts. On the other hand, if the current collector has athickness of more than 40 μm, the volume ratio of the current collectorto the battery increases, which is disadvantageous in terms of capacitydepending on the kind of the battery. Also, thick current collectors aredifficult to handle, for example, difficult to bend.

The negative electrode mixed material layer comprises a mixture of thealloying material, the graphite, the binder and the like, and itcontains, if necessary, other additives, such as an auxiliary conductiveagent. The thickness of the negative electrode mixed material layer onone side of the current collector is generally not less than 10 μm andnot greater than 100 μm, and it is often not less than 50 μm and notmore than 100 μm. Although the thickness of the mixed material layer maybe less than 10 μm, care should be taken to see that the volume ratio ofthe current collector to the negative electrode does not become toolarge. Also, the thickness of the mixed material mixture layer may bemore than 100 μm, but an electrolyte may not penetrate through to thevicinity of the current collector, thereby resulting in a degradation inhigh-rate discharge characteristics.

The density of the negative electrode mixed material layer is preferably0.8 g/cm³ to 2 g/cm³ in a discharged state. The porosity of the negativeelectrode mixed material layer is desirably 70% or less. The porosity iscalculated as follows.(the measured density of the negative electrode mixed materiallayer)/(the true density of the negative electrode mixed materiallayer)×100(%)

The true density of the negative electrode mixed material layer iscalculated from the respective true densities of the raw materials(alloying material, graphite, binder and the like) of the negativeelectrode mixed material and the mixing ratio thereof.

The non-aqueous electrolyte secondary battery according to the presentinvention includes the above-described negative electrode, a positiveelectrode capable of electrochemically absorbing and desorbing Li, and anon-aqueous electrolyte.

The positive electrode may be any positive electrode that isconventionally suggested or disclosed in related arts, without anyparticular limitation. The positive electrode may be produced byconventional methods. For example, the positive electrode can beobtained by mixing a positive electrode active material, a conductiveagent such as carbon black and a binder such as polyvinylidene fluoridetogether in a liquid phase, applying the resultant paste onto a positiveelectrode current collector made of Al or the like, and drying androlling it.

The positive electrode active material may be any positive electrodeactive material that is conventionally suggested or disclosed in relatedarts, without any particular limitation. However, lithium containingtransition metal compounds are preferable. Typical examples of thelithium containing transition metal compounds include, but are notlimited to, LiCoO₂, LiNiO₂, LiMn₂O₄, and LiMnO₂. Compounds obtained byreplacing the transition metal element of the above-mentioned compoundswith a different metal element are also used preferably. Examplesinclude LiCo_(1-x)Mg_(x)O₂, LiNi_(1-y)CO_(y)O₂, andLiNi_(1-y-z)Co_(y)Mn_(z)O₂ (x, y, and z are positive).

The non-aqueous electrolyte may be any electrolyte that isconventionally suggested and disclosed in related arts, without anyparticular limitation. However, an electrolyte comprising a non-aqueoussolvent and a lithium salt that is soluble therein is preferred. Acommon non-aqueous solvent is a solvent mixture of a cyclic carbonate,such as ethylene carbonate or propylene carbonate, and a chaincarbonate, such as dimethyl carbonate, diethyl carbonate, or ethylmethyl carbonate. Further, γ-butyrolactone, dimethoxyethane or the likemay be mixed in a non-aqueous solvent. Also, examples of the lithiumsalt include inorganic lithium fluorides and lithium imide compounds.The former examples include LiPF₆ and LiBF₄, and the latter examplesinclude LiN(CF₃SO₂)₃. Further, LiClO₄, LiCF₃SO₃ or the like may be mixedin the lithium salt. The non-aqueous electrolyte may be a gelelectrolyte or a solid electrolyte.

A separator is placed between the positive electrode and the negativeelectrode in order to prevent internal short-circuits therebetween. Theseparator may be made of any material that allows the non-aqueouselectrolyte to pass through to a suitable extent and prevents thecontact between the positive electrode and the negative electrode. Amicroporous film made of polyethylene, polypropylene or the like isgenerally used in non-aqueous electrolyte secondary batteries, and thethickness thereof is generally not less than 10 μm and not more than 30μm.

The present invention is applicable to non-aqueous electrolyte secondarybatteries of various shapes, such as cylindrical, flat, coin, andrectangular shapes, and the battery shape is not particularly limited.The present invention is applicable to various sealing types ofbatteries, including batteries in which power generating elements, suchas electrodes and an electrolyte, are accommodated in a metal batterycase or a case made of a laminated film. There is no particularlimitation with respect to how the batteries are sealed.

In the following, the present invention will be specifically describedby way of Examples and Comparative Examples. The following Examples,however, only show preferable embodiments of the present invention, andhence, the present invention is not limited to the following Examples.

EXAMPLE 1

In Examples and Comparative Examples, negative electrodes andcylindrical batteries were produced in the following manner, and theircycle life and discharge capacity were evaluated.

(1) Alloying Material Preparation

Ti metal with a purity of 99.9% and a particle size of 100 to 150 μm wasmixed with Si with a purity of 99.9% and a mean particle size of 3 μm ina weight ratio of Ti:Si=9.2:90.8.

This powder mixture of 3.5 kg was placed in the container of a vibrationmill (FV-20, manufactured by Chuo Kakohki Co., Ltd.). Then,2-cm-diameter stainless steel balls were placed therein such that theyoccupied 70% of the internal volume of the container. After thecontainer was evacuated, Ar (purity 99.999%, manufactured by NipponSanso Corporation) was introduced thereinto so as to provide a pressureof 1 atmosphere. The operating conditions of the mill were: amplitude of8 mm and revolution frequency of 1200 rpm. Under such conditions, themechanical alloying operation was performed for 80 hours.

The Ti—Si alloy obtained by the above operation was removed from thecontainer, and its particle size distribution was examined. It was foundthat this alloy had a wide particle size distribution of 0.5 μm to 80μm. The Ti—Si alloy was classified with a sieve (10 μm mesh size), whichgave an alloying material (hereinafter referred to as “alloy a”) havinga maximum particle size of 10 μm and a mean particle size of 8 μm.

The alloying material a was measured by X-ray diffraction analysis, andan XRD profile as shown in FIG. 2 was obtained. As can be seen from FIG.2, the alloying material a was a microcrystalline alloying material, andthe size of its crystal grains (crystallites) calculated from the halfwidth of the most intensive peak based on the Scherrer equation was 10nm.

In the XRD profile, the maximum peak appeared around 20=28 to 290, andthe half width of the peak was 0.50. The half width was calculated fromFIG. 3, which was obtained by removing the background from FIG. 2.

The result of the X-ray diffraction analysis indicated that a Si-onlyphase (A phase) and a TiSi₂ phase (B phase) were present in the alloyingmaterial a. When the ratio between the Si-only phase and the TiSi₂ phasewas calculated on the assumption that the alloying material a wascomposed of these two phases only, it was found that Si:TiSi₂=80:20(weight ratio).

A section of the alloying a was observed with a transmission electronmicroscope (TEM). As a result, it was found that the Si-only phase,composed of an amorphous region and crystal grains (crystallites) with agrain size of approximately 10 nm, and the TiSi₂ phase, composed ofcrystal grains (crystallites) with a grain size of approximately 15 to20 nm, were present.

(2) Negative Electrode Preparation

The alloying material a was mixed with graphite in weight ratios aslisted in Table 1. A negative electrode material mixture paste wasprepared by adding 5 parts by weight of polyacrylic acid (molecularweight: 150,000, manufactured by Wako Pure Chemical Industries, Ltd.) asa binder per 100 parts by weight of the total of the alloy a and thegraphite, and sufficiently kneading the mixture with pure water.Therein, the whole amount of the alloying material a and 2.5 parts byweight of polyacrylic acid were kneaded until they became homogeneous,and then, the graphite and the remaining polyacrylic acid were added tothe mixture and kneaded.

The graphite used was flake graphite (KS-44) with a mean particle sizeof 20 μm, manufactured by Timcal Ltd.

The negative electrode material mixture paste was applied to both sidesof a current collector, made of a 10-μm-thick electrolytic copper foil(manufactured by Furukawa Circuit Foil Co., Ltd). The current collectorapplied with the paste was then dried and rolled, which gave a negativeelectrode comprising the current collector and a negative electrodematerial mixture layer carried on each side of the current collector.

A section of the negative electrode thus obtained was observed with ascanning electron microscope (SEM), and it was found that the structurethereof was almost the same as that of FIG. 1. The density of thenegative electrode material mixture layer was 1.3 to 1.4 g/cm³, and theporosity of the negative electrode material mixture layer was 40 to 45%.

(3) Positive Electrode Preparation

LiCoO₂, which is a positive electrode active material, was synthesizedby mixing Li₂CO₃ and COCO₃ in a predetermined molar ratio, and heatingthe resultant mixture at 950° C. The synthesized LiCoO₂ was classifiedinto a size of 45 μm or less. 100 parts by weight of the LiCoO₂(positive electrode active material) was fully mixed with 5 parts byweight of acetylene black serving as a conductive agent, 4 parts byweight of polyvinylidene fluoride as a binder, and a proper amount ofN?methyl-2-pyrrolidone as a dispersion medium, to obtain a positiveelectrode material mixture paste.

The positive electrode material mixture paste was applied on both sidesof a current collector, made of a 15-μm-thick aluminum foil(manufactured by Showa Denko K.K.). The resultant current collectorapplied with the paste was then dried and rolled, which gave a positiveelectrode comprising the current collector and a positive electrodematerial mixture layer carried on each side of the current collector.

(4) Cylindrical Battery Preparation

A cylindrical lithium ion secondary battery as illustrated in FIG. 4 wasproduced.

A positive electrode 35 and a negative electrode 36 were cut into apredetermined size. One end of an aluminum positive electrode lead 35 awas connected to the current collector of the positive electrode. Oneend of a nickel negative electrode lead 36 a was connected to thecurrent collector of the negative electrode. Thereafter, the positiveelectrode 35 and the negative electrode 36 were rolled up with aseparator 37 comprising a polyethylene resin microporous film, which is20 μm in thickness and wider than the electrode plates, interposedtherebetween, to produce an electrode plate group. The outer face ofthis electrode plate group was covered with the separator 37. An upperinsulating ring 38 a and a lower insulating ring 38 b were fitted to theupper and lower parts of the electrode plate group, which was thenplaced into a battery case 31. Subsequently, a non-aqueous electrolytewas injected into the battery case to impregnate the electrode plategroup therewith. The other end of the positive electrode lead 35 a waswelded to the backside of a sealing plate 32, whose circumference wasfitted with an insulating gasket 33. The other end of the negativeelectrode lead 36 a was welded to the inner bottom face of the batterycase. Lastly, the opening of the battery case 31 was closed with thesealing plate 32. In this way, cylindrical lithium ion secondarybatteries 1 to 9 were completed. Of the batteries 1 to 9, the batteries1 to 6 are Examples of the present invention, and the batteries 7 to 9are Comparative Examples.

The non-aqueous electrolyte used was prepared by dissolving lithiumhexafluorophosphate (LiPF₆) at a concentration of 1 mol/L in anon-aqueous solvent mixture of ethylene carbonate and diethyl carbonatein a volume ratio of 1:1.

(5) Battery Evaluation

(i) Discharge Capacity

In a constant temperature room of 20%, each cylindrical battery wascharged at a constant charge current of 0.2 C (1 C is 1 hour-ratecurrent) until the battery voltage reached 4.05 V, and then charged at aconstant voltage of 4.05 V until the current value reached 0.01 C.Thereafter, the cylindrical battery was discharged at a current of 0.2 Cuntil the battery voltage dropped to 2.5 V. Table 1 shows the dischargecapacity.

(ii) Cycle Life

After the above-mentioned measurement of the discharge capacity, thefollowing charge/discharge cycle of the battery was repeated in aconstant temperature room at 20° C.

Each battery was charged at a constant charge current of 1 C until thebattery voltage reached 4.05 V, and then charged at a constant voltageof 4.05 V until the current value reached 0.05 C. Thereafter, thecylindrical battery was discharged at a current of 1 C until the batteryvoltage dropped to 2.5 V. This cycle was repeated. The ratio of thedischarge capacity at the 100th cycle to the discharge capacity at thesecond cycle was expressed in percentage, which was defined as capacitymaintenance rate (%). Table 1 shows the results. The closer the capacitymaintenance rate is to 100%, the better the cycle life is. TABLE 1Graphite Alloying Discharge Capacity (% by Material capacity maintenanceBattery weight) (% by weight) (mAh) rate (%) 1 95 5 1550 95 2 90 10 160095 3 80 20 1660 95 4 70 30 1720 94 5 60 40 1790 89 6 50 50 1870 83 7 982 1100 96 8 45 55 1320 95 9 30 70 1920 65

COMPARATIVE EXAMPLE 1

A negative electrode was produced by using graphite alone without usingthe alloying material a. A cylindrical lithium ion secondary battery 10was produced in the same manner as in Example 1 except for the use ofthis negative electrode. Therein, 5 parts by weight of the binder(polyacrylic acid) was added per 100 parts by weight of graphite.

COMPARATIVE EXAMPLE 2

A negative electrode was produced by using the alloying material alonewithout using graphite and using, as an auxiliary conductive agent, 10parts by weight of acetylene black (DENKA BLACK, manufactured by DenkiKagaku Kogyo K.K.) having a specific surface area of 70 m²/g per 100parts by weight of the alloying material a. A cylindrical lithium ionsecondary battery 11 was produced in the same manner as in Example 1except for the use of this negative electrode. Therein, 5 parts byweight of the binder (polyacrylic acid) was added per 100 parts byweight of the alloying material a.

The batteries of Comparative Examples 1 and 2 were evaluated in the samemanner as in Example 1. Table 2 shows the results. TABLE 2 GraphiteAlloying Discharge Capacity (% by Material capacity maintenance Batteryweight) (% by weight) (mAh) rate (%) 10 100 0 1030 96 11 0 100 1980 34

It can be seen that the batteries 1 to 6 of Example 1, using a negativeelectrode in which the weight percentage of the graphite relative to thetotal weight of the alloy a and the graphite is 50% to 95%, haveimproved capacities, particularly compared with Comparative Example 1,and have improved cycle lives compared with Comparative Example 2.

Also, the batteries of Comparative Examples 1 and 2 were disassembledafter 100 cycles and evaluated for the degree of expansion. As a result,it was found that the negative electrode of Comparative Example 1expanded 1.1-fold and the negative electrode of Comparative Example 2expanded 3.2-fold, as compared with the thickness of the negativeelectrode before the charge/discharge. On the other hand, for example,the battery 3 of Example 1 was disassembled after 100 cycles andevaluated for the degree of expansion. As a result, an approximately1.5-fold expansion was observed. That is, it was found that thebatteries of Example were able to suppress the expansion due to thealloying while maintaining high capacity.

EXAMPLE 2

In producing a negative electrode, the mixing weight ratio of graphite(KS-44, manufactured by Timcal Ltd.) and the Ti—Si alloy (alloy a) wasfixed at 80:20. Also, the above-mentioned acetylene black (DENKA BLACK,manufactured by Denki Kagaku Kogyo K.K., with a specific surface area of70 m²/g) or graphite with a specific surface area of 14 m²/g (KS4,manufactured by Timcal Ltd.) was added as an auxiliary conductive agent,in parts by weight as listed in Table 3 per 100 parts by weight of thetotal of the alloying material a and the graphite (KS-44). As thebinder, 5 parts by weight of polyacrylic acid was added.

Except for the above, in the same manner as in Example 1, cylindricallithium ion secondary batteries 12 to 20 were produced. Therein, thewhole amount of the alloying material a, 3 parts by weight ofpolyacrylic acid and a predetermined amount of acetylene black or KS4were kneaded until they became homogeneous, and then, the graphite(KS-44) and the remaining polyacrylic acid were added and kneaded.

The batteries 12 to 20 of Example 2 were evaluated in the same manner asin Example 1. Table 3 shows the results. TABLE 3 Auxiliary conductiveagent Discharge Capacity Amount capacity maintenance Battery Kind (partby weight) (mAh) rate (%) 12 Acetylene black 2 1750 96 13 Acetyleneblack 5 1690 95 14 Acetylene black 10 1590 95 15 Acetylene black 15 113096 16 Graphite (KS4) 2 1770 94 17 Graphite (KS4) 5 1700 93 18 Graphite(KS4) 10 1620 93 19 Graphite (KS4) 15 1500 90 20 Graphite (KS4) 30 103085

In the batteries of this Example, in which their negative electrodesincluded acetylene black or KS4 as the auxiliary conductive agent, theircharacteristics improved, and particularly their cycle life improvedremarkably. Also, even when the amount of the auxiliary conductive agentwas approximately 2 parts by weight, which is small, the capacityincreased, in comparison with the case where no auxiliary conductiveagent was added (battery 3 of Example 1).

A section of each negative electrode of this example was observed withan SEM, and a proper amount of the auxiliary conductive agent was foundto be around the particles of the alloying material a. It is consideredthat such arrangement of the auxiliary conductive agent ensuredsufficient conductivity of the negative electrode, thereby enabling thealloying material a to exhibit its maximum capacity.

EXAMPLE 3

(i) Battery 21

The Ti—Si alloying material with a wide particle size distribution of0.5 μm to 80 μm, which was obtained in the same manner as in Example 1,was put through a first sieve (45 μm mesh size) to remove particleslarger than 45 μm. The alloy was then put through a second sieve (20 μmmesh size) to remove particles smaller than 20 μm, to obtain an alloymaterial (hereinafter referred to as alloying material b) with aparticle size distribution of 20 to 45 μm and a mean particle size of 32μm.

A cylindrical lithium ion secondary battery 21 was produced in the samemanner as the battery 3 of Example 1 except for the use of the alloyingmaterial b.

(ii) Battery 22

The Ti—Si alloying material with a wide particle size distribution of0.5 μm to 80 μm, which was obtained in the same manner as in Example 1,was put through a first sieve (20 μm mesh size) to remove particleslarger than 20 μm. The alloying material was then put through a secondsieve (10 μm mesh size) to remove particles smaller than 10 μm, toobtain an alloying material (hereinafter referred to as alloyingmaterial c) with a particle size distribution of 10 to 20 μm and a meanparticle size of 13 μm.

A cylindrical lithium ion secondary battery 22 was produced in the samemanner as the battery 3 of Example 1 except for the use of the alloyingmaterial c.

(iii) Battery 23

The Ti—Si alloying material with a wide particle size distribution of0.5 μm to 80 μm, which was obtained in the same manner as in Example 1,was put through a sieve (10 μm mesh size) to remove particles largerthan 10 μm, to obtain an alloying material (hereinafter referred to asalloying material d) with a maximum particle size of 10 μm and a meanparticle size of 8 μm.

A cylindrical lithium ion secondary battery 23 was produced in the samemanner as the battery 3 of Example 1 except for the use of the alloyingmaterial d.

The batteries 21 to 23 of Example 3 were evaluated in the same manner asin Example 1. Table 4 shows the results. TABLE 4 Mean Maximum DischargeCapacity particle particle capacity maintenance Battery size (μm) size(μm) (mAh) rate (%) 21 32 44 1690 18 22 13 18 1700 26 23 8 10 1710 60

The batteries of this example had large initial capacities, but theircharge/discharge cycle characteristics tended to be low. In the case ofthe batteries 21 and 22, in particular, the results were not as good aseven those of Comparative Example 2. A section of the negative electrodeof each of the batteries 21 to 22 was observed with an SEM, and it wasfound that the negative electrode had a structure different from that asshown in FIG. 1, with graphite particles and particles of alloyingmaterial agglomerating separately.

The batteries 21 and 22 were disassembled after 100 cycles, and theirnegative electrodes were observed. As a result, it was found that mostof the material mixture separated from and fell off the currentcollector. Further, these current collectors were observed to bewrinkled and cracked at the edge thereof. This is probably because thestress exerted by the expansion of the alloy material deformed thecurrent collectors, thereby leading to defects such as wrinkles andcracking. Also, in the battery 23, partial separation of the materialmixture was observed.

COMPARATIVE EXAMPLE 3

The Ti—Si alloying material (alloying material a) used in Example 1 wasintroduced into an electric furnace and heat-treated at 1000° C. in avacuum for 3 hours. As a result, the alloying material a turned into ahighly crystalline alloying material (hereinafter referred to asalloying material e). The alloying material e was measured by X raydiffraction analysis, and the size of its crystal grains (crystallites)calculated from the half width of the most intensive peak based on theScherrer equation was 1 μm. However, the particle size distribution ofthe alloying material e remained unchanged and the same as that of thealloying material a. Thus, the mean particle size of the alloyingmaterial e was 8 μm, and its maximum particle size was 10 μm.

A cylindrical lithium ion secondary battery 24 was produced in the samemanner as the battery 3 of Example 1 except for the use of the alloyingmaterial e, and then evaluated in the same manner as in Example 1. Table5 shows the results. TABLE 5 Crystal grain Discharge Capacitymaintenance Battery size (μm) capacity (mAh) rate (%) 24 1 1640 11

The battery 24 has a high capacity, but its charge/discharge cyclecharacteristics are low. The battery 24 was disassembled after 100cycles, and its negative electrode was observed. It was found that theparticles of alloying material were further pulverized into sub-micronsize. The reason is considered to be that the Si crystal phase, whichwas produced by the heat-treatment, was expanded and destroyed by theabsorption of Li.

EXAMPLE 4

An alloying material was produced by the same synthesis method as thatof Example 1, except for the use of Zr, Ni, Co, Mn, Fe or Cu (purity:99.9%, particle size: 100 to 150 μm) as the transition metal M insteadof metal Ti. The resultant alloying material was put through a sievewhich was the same as that of Example 1, to obtain an alloying materialwith a maximum particle size of 8 μm and a mean particle size of 5 μm.Alloying materials using Zr, Ni, Co, Mn, Fe and Cu were named alloyingmaterials f, g, h, i, j and k, respectively.

The alloying materials f to k were measured by X ray diffractionanalysis, and the resultant XRD profiles were similar to that of FIG. 2,showing that these alloying materials were microcrystalline. Also, thesize of crystal grains (crystallites) of each alloying materialcalculated from the half width of the most intensive peak based on theScherrer equation was in the range of 9 to 25 nm.

The results of the X-ray diffraction analysis indicated that a Si-onlyphase (A phase) and a MSi₂ phase (B phase) were present in the alloyingmaterials f to k. On the assumption that each of the alloying materialsf to k was composed of these two phases only, the ratio between theSi-only phase and the MSi₂ phase was calculated. The MSi₂:Si weightratios are shown in Table 6 (Si phase: 65 to 83% by weight).

Cylindrical lithium ion secondary batteries 25 to 30 were produced inthe same manner as the battery 3 of Example 1 except for the use of thealloying materials f to k, and they were evaluated in the same manner asin Example 1. Table 6 shows the results. TABLE 6 Transition MSi₂:SiDischarge Capacity metal (weight capacity maintenance Battery element Mratio) (mAh) rate (%) 25 Zr 25:75 1690 93 26 Ni 31:69 1650 89 27 Co35:65 1630 90 28 Mn 22:78 1720 87 29 Fe 17:83 1740 88 30 Cu 28:72 169086

The results of the batteries 25 to 30 indicate that the use of any ofthe alloying materials results in a battery having both high capacityand long cycle life. It is noted that the Ti—Si alloy (alloy a) and theZr—Si alloy (alloy f) exhibited particularly good charge/discharge cyclecharacteristics. This is probably because the particles of thesealloying materials have higher electronic conductivity than otheralloying materials, thereby providing good charge/discharge cyclecharacteristics without being influenced by expansion.

EXAMPLE 5

The Ti—Si alloying material with a wide particle size distribution of0.5 μm to 80 μm, which was obtained in the same manner as in Example 1,was classified with various sieves, to obtain alloying materials eachhaving a mean particle size of 2 μm, 5 μm or 7 μm and a maximum particlesize of not larger than 10 μm.

Meanwhile, graphite (KS-44) was crushed and classified with sieves, toobtain graphite materials each having a mean particle size of 8 μm, 13μm, 16 μm or 20 μm (uncrushed).

The alloying material and the graphite material thus obtained were mixedtogether in the combinations as listed in Table 7 such thatgraphite:alloy=80:20 (weight ratio). Except for this, in the same manneras the battery 3 of Example 1, cylindrical lithium ion secondarybatteries 31 to 38 were produced, and they were evaluated in the samemanner as in Example 1. Table 7 shows the results as well as the ratioof the mean particle size of the alloying material (R alloy) to the meanparticle size of the graphite (R graphite), i.e., R alloy/R graphite.TABLE 7 Alloying Graphite Material mean mean Discharge Capacity particleparticle R_(alloy)/ capacity maintenance Battery size (μm) size (μm)R_(graphite) (mAh) rate (%) 31 13 2 0.15 1590 81 32 13 5 0.38 1660 88 3313 7 0.54 1700 84 34 8 2 0.25 1540 89 35 8 7 0.88 1430 70 36 16 2 0.131550 48 37 16 5 0.31 1730 84 38 20 2 0.10 1560 37

The batteries 36 and 38 had relatively short cycle lives. The R alloy/Rgraphite values of the batteries 36 and 38 were lower than 0.15. Asection of the negative electrodes of these batteries was observed withan SEM, and it was found that a large number of particles of alloyingmaterial were squeezed and agglomerated between graphite particles.Therefore, when these batteries were disassembled after 100 cycles toobserve their negative electrodes, part of the negative electrodematerial mixture was found to be separated from the current collector.

On the other hand, when the R alloy/R graphite value was in the range of0.15 to 0.9, high capacity and good cycle life were obtained. Also, whenthe R alloy/R graphite value was in the range of 0.2 to 0.4,particularly high capacity and long cycle life were obtained. A sectionof the negative electrodes of these batteries was observed, and it wasfound that these negative electrodes had a structure as shown in FIG. 1,with particles of alloying material evenly dispersed between graphiteparticles.

EXAMPLE 6

In the same manner as the batteries 1 to 6 of Example 1, the alloy a andgraphite (KS-44) were used in the weight ratios as listed in Table 8.Also, 5 parts by weight of polyacrylic acid (molecular weight: 150,000,manufactured by Wako Pure Chemical Industries, Ltd.) serving as a binderwas used per 100 parts by weight of the total of the alloy a and thegraphite. However, the whole amounts of graphite and polyacrylic acidwere sufficiently kneaded with pure water in advance. Thereafter, thealloy a was added to the mixture of graphite, polyacrylic acid and purewater, and the mixture was further kneaded to form a negative electrodematerial mixture paste. A negative electrode was produced in the samemanner as in Example 1 except for the use of this paste.

A section of the negative electrode thus obtained was observed with anSEM, and it was confirmed that the structure thereof was almost the sameas that of FIG. 1. Further, it was confirmed that the graphite surfaceand the alloy a were adhered at a plurality of contact points. Thedensity of the negative electrode material mixture layer was 1.3 g/cm³,and the porosity of the negative electrode material mixture layer was45%.

Using the electrodes thus obtained, cylindrical lithium ion batteries(batteries 39 to 44) were produced in the same manner as in Example 1.The batteries 39 to 44 were evaluated in the same manner as inExample 1. Table 8 shows the results. TABLE 8 Graphite Alloy DischargeCapacity maintenance Battery (wt %) (wt %) capacity (mAh) rate (%) 39 955 1560 97 40 90 10 1620 96 41 80 20 1670 96 42 70 30 1730 95 43 60 401800 91 44 50 50 1890 86

The batteries 39 to 44 exhibited slightly higher capacities and improvedcycle characteristics in comparison with the batteries 1 to 6 ofExample 1. After the evaluation, these batteries were disassembled andexamined. As a result, it was found that the negative electrodeexpansion was 1.3- to 1.4-fold, which was low. Also, the batteries 4 and42 were disassembled and their negative electrodes were observed with anSEM. As a result, in the negative electrode of the battery 4, a part ofthe mixture layer surface was bulged. On the other hand, in the battery42, the surface was almost flat and smooth. In the batteries of Example6, the alloying material was fixed to the graphite surface via thebinder, and this is thought to be the reason why excessive expansion wassuppressed.

EXAMPLE 7

(Battery 45)

A negative electrode was produced in the same manner as the battery 4 ofExample 1, except that 3 parts by weight of VGCF (average length: 20 μm,aspect ratio: 500, manufactured by Showa Denko K.K.) was added as thecarbon fibers per 100 parts by weight of the total of the alloy a andthe graphite. Using this negative electrode, a battery 45 was producedin the same manner as in Example 1.

(Battery 46)

The alloy a was mounted on a SiO₂ boat, which was then placed in atubular furnace. The interior of the furnace was maintained at a vacuumof 3.0×10⁻¹ Pa. Methane was circulated together with a mixed gas ofhelium and hydrogen inside the vacuum furnace at a flow rate of 10 sccm.In this state, the alloy a was heated at 500° C. for 30 minutes. As aresult, carbon fibers with an aspect ratio of approximately 20 to 100were successfully grown on the surface of the alloy a. The amount ofcarbon fibers produced was 6 parts by weight per 100 parts by weight ofthe alloy a. A negative electrode was produced in the same manner as thebattery 4 of Example 1 except for the use of the composite material ofthe alloy a and carbon fibers thus obtained instead of the alloy a.Using this negative electrode, a battery 46 was produced in the samemanner as in Example 1.

(Battery 47)

Graphite and the alloy a were mixed in such a weight ratio thatgraphite:alloy a=70:30 and then formed into a composite material by amechanofusion device (manufactured by Hosokawa Micron Corporation). Whenthe resultant composite material was observed with an SEM, it was foundthat the graphite surface was covered with the alloy a. The meanparticle size of the composite material was approximately 28 μm. Carbonfibers were grown on the surface of the composite material under thesame conditions of the battery 48 except for the use of this compositematerial instead of the alloy a. An SEM observation revealed that theaspect ratio of the carbon fibers was approximately 20 to 100 and that apart of the carbon fibers bonded to not only the surface of the alloy abut also the graphite surface. The amount of carbon fibers produced was6 parts by weight per 100 parts by weight of the alloy a. A negativeelectrode was produced in the same manner as the battery 4 of Example 1except for the use of the composite material of the alloy a, graphiteand carbon fibers thus obtained instead of the alloy a and graphite.Using this negative electrode, a battery 47 was produced in the samemanner as in Example 1.

The batteries 45 to 47 were evaluated in the same manner as inExample 1. Table 9 shows the results. TABLE 9 Discharge Capacitymaintenance Battery Condition capacity (mAh) rate (%) 45 Addition ofVGCF 1720 96 46 Forming composite 1700 97 material of alloy and carbonfibers 47 Forming composite 1690 97 material of alloy, carbon fibers andgraphite

As shown in Table 9, the batteries 45 to 47 exhibited improvedcharge/discharge cycle characteristics, as compared with the battery 4.That is, the addition of carbon fibers to the negative electrode waseffective in improving the charge/discharge cycle characteristics. Also,when carbon fibers were bonded to the alloying material or graphite, theimprovement in charge/discharge cycle characteristics was greater. Thisis probably because the current collecting property was improved.

As described above, the negative electrode for a non-aqueous electrolytesecondary battery in accordance with the present invention provides anexcellent non-aqueous electrolyte secondary battery having both highcapacity and good charge/discharge cycle characteristics. The presentinvention is applicable to non-aqueous electrolyte secondary batteriesin any form. For example, the present invention is applicable not onlyto cylindrical batteries described in Examples, but also to batteries ofcoin, rectangular, or flat shape with a wound or layered electrode plategroup. The non-aqueous electrolyte secondary battery of the presentinvention is useful as a main power source of mobile communicationsappliances or portable electronic appliances.

Although the present invention has been described in terms of thepresently preferred embodiments, it is to be understood that suchdisclosure is not to be interpreted as limiting. Various alterations andmodifications will no doubt become apparent to those skilled in the artto which the present invention pertains, after having read the abovedisclosure. Accordingly, it is intended that the appended claims beinterpreted as covering all alterations and modifications as fall withinthe true spirit and scope of the invention.

1. A negative electrode for a non-aqueous electrolyte secondary battery,comprising graphite and at least one alloying material capable ofelectrochemically absorbing and desorbing Li, said alloying materialcomprising an A phase composed mainly of Si and a B phase comprising anintermetallic compound of at least one transition metal element and Si,at least one of said A phase and said B phase comprising amicrocrystalline or amorphous region, the weight percentage of said Aphase relative to the total weight of said A phase and said B phasebeing greater than 40% and not greater than 95%, the weight percentageof said graphite relative to the total weight of said alloy material andsaid graphite being not less than 50% and not greater than 95%.
 2. Thenegative electrode for a non-aqueous electrolyte secondary battery inaccordance with claim 1, wherein said alloying material exists in gapsbetween particles of said graphite.
 3. The negative electrode for anon-aqueous electrolyte secondary battery in accordance with claim 2,wherein said alloying material has a maximum particle size of 10 μm orless.
 4. The negative electrode for a non-aqueous electrolyte secondarybattery in accordance with claim 1, further comprising a binder, whereinat least a part of said alloying material is adhered to the surface ofsaid graphite via the binder.
 5. The negative electrode for anon-aqueous electrolyte secondary battery in accordance with claim 1,wherein the ratio of the mean particle size of said alloying material tothe mean particle size of said graphite is in the range of 0.15 to 0.90.6. The negative electrode for a non-aqueous electrolyte secondarybattery in accordance with claim 1, further comprising an auxiliaryconductive agent, said auxiliary conductive agent having a specificsurface area of 10 m²/g or more.
 7. The negative electrode for anon-aqueous electrolyte secondary battery in accordance with claim 6,wherein said auxiliary conductive agent comprises carbon fibers havingan aspect ratio of 10 or more, and at least one end of said carbonfibers is adhered or bonded to said alloying material.
 8. The negativeelectrode for a non-aqueous electrolyte secondary battery in accordancewith claim 7, wherein one end of at least a part of said carbon fibersis adhered or bonded to said alloying material while the other end isadhered or bonded to said graphite.
 9. The negative electrode for anon-aqueous electrolyte secondary battery in accordance with claim 8,wherein said carbon fibers are obtained by heating at least one of saidalloying material and said graphite in the flow of hydrocarbon gas. 10.The negative electrode for a non-aqueous electrolyte secondary batteryin accordance with claim 5, wherein the weight percentage of saidauxiliary conductive agent relative to the total weight of said alloyingmaterial, said graphite and said auxiliary conductive agent is 10% orless.
 11. A non-aqueous electrolyte secondary battery comprising apositive electrode capable of electrochemically absorbing and desorbingLi, a negative electrode and a non-aqueous electrolyte, said negativeelectrode comprising graphite and at least one alloying material capableof electrochemically absorbing and desorbing Li, said at least onealloying material comprising an A phase composed mainly of Si and a Bphase comprising an intermetallic compound of at least one transitionmetal element and Si, at least one of said A phase and said B phasecomprising a microcrystalline or amorphous region, the weight percentageof said A phase relative to the total weight of said A phase and said Bphase being greater than 40% and not greater than 95%, the weightpercentage of said graphite relative to the total weight of saidalloying material and said graphite being not less than 50% and notgreater than 95%.